The aim of this study is to determine the contribution of intramuscular vs circulating sources of lipid and carbohydrate to exercise fuel utilization in males and females and to determine the post-exercise changes in hormone and fuel metabolism. It has previously been shown that females utilize proportionally more fat, whereas males utilize proportionally more carbohydrate, during moderate exercise. The combination of indirect calorimetry and stable isotope techniques will be used to determine potential gender differences in the contribution of intramuscular vs circulating fuels to whole body lipid and carbohydrate oxidation. Simultaneously, the circulating hormone environment will be determined to ascertain any relationship with patterns of nutrient utilization. Subjects follow 3 days of controlled diet and are studied in the morning after an overnight fast. Following resting measurements, cycle exercise is performed for 90 minutes, at a steady state intensity of 90% lactate threshold. Subjects then rest for 3 hours with measurements continuing throughout the post-exercise period. It is hypothesized that a) the greater proportional fat oxidation in females will be due to a greater utilization of intramuscular triglyceride (and possibly very-low density lipoprotein triglyceride) b) the greater proportional carbohydrate oxidation in males will be due to a greater utilization of both muscle glycogen and blood glucose c) post-exercise circulating FFA oxidation will be greater in males whereas FFA re-esterification will be greater in females. In this way, females may minimize both utilization of peripheral lipid stores and endogenous glucose. This has implications for gender differences in the mechanisms controlling body fat stores and lipid/carbohydrate interactions.